~ miscellanea ~

- 2 -
Kalendae, Nonae, Idus

ancient Rome's dating systems

Whenever you check a date on a calendar, you are looking at something that comes from ancient Rome.
The dating system presently adopted in most countries of the world, as well as many names too, such as those of months and weekdays, are largely based on the ones that were already in use well over 2000 years ago; the same word "calendar" comes from Kalendae, indicating the first day of each month. Some details of this system, though, would now appear very complicated and unpractical.
This page describes how the archaic systems developed into the present one, and its many connections with the city of Rome.

A very early attempt of dividing the time elapsed between the cyclic changing of the seasons was a year comprising ten months, whose total length was 304 days, and started on the full moon of the month. This scheme, likely of Greek origin, was already in use by the time of Rome's foundation, but it was obviously inaccurate; for this reason, an uncounted number of days was likely added, so that events for which a specific timing was required, such as sowing or gathering the harvest, could be carried out reasonably on time.

The second Roman king, Numa Pompilius (c.700 BC), is traditionally credited for having increased the number of months by two, thus lengthening the year to 355 days. Having this scheme been adopted also during the following Republican Age, up to the 1st century BC, it is now usually referred to as the Roman Republican year.

This year originally started with the month of March (New Year's Day fell on March 14), probably because early spring is the period in which nature comes to new life again after the cold season. January and February, which had been added shortly earlier, were in fact the last months of the year.
Half of the twelve months had names that recalled the gods whom they were sacred to, while others were named after their ordering sequence:

Martius Mars - god of war 31 days March 31 days
Aprilis Venus (Greek: Aphrodite) - goddess of beauty
and love; maybe also after verb aperire, "to open",
referring to the unfolding of buds in spring
29 days April 30 days
Maius Maia - one of the seven Pleiades 31 days May 31 days
Iunius Juno - queen of all gods 29 days June 30 days
Quintilis the fifth month 31 days July 30 days
Sextilis the sixth month 29 days August 31 days
September the seventh month 29 days September 30 days
October the eighth month 31 days October 31 days
November the ninth month 29 days November 30 days
December the tenth month 29 days December 31 days
Ianuarius Janus - god of gates and beginnings 29 days January 31 days
Februarius Februus - minor Roman god, later identified with Pluto, ruler of the infernal regions 28 days February 28 days
Total length of the year : 355 days 365 days

Latin names of the months are adjectives that express a dedication or a belonging (e.g. Martius = "month of Mars", etc.).

The fifth Roman king, Tarquinius Priscius (c.600 BC) decided that January should have been the opening month of the year, because sacred to the god of all beginnings. But this change did not last long: when the Etruscan dynasty he belonged to was dismissed about one century later, March became again the first month, according to Latin tradition.

The 355-day system, though, kept causing a certain mismatch between different years, due to its short duration, and the problem was coped with by adopting a thirteenth month, named Intercalans ("intercalary month") or Mercedonius ("month of wages", because by this time of the year workers received their payment); its length was either 27 or 28 days. About once every two years, the additional month was inserted between February 23rd and 24th, and the five remaining days of the month were cancelled.
The pontefices, a body of officers who presided over religious celebrations, decided when the Intercalans had to be adopted, and how long it would have lasted. Their head was the pontifex maximus, a charge held by the emperor himself up to the early Christian age. The standard length of the intercalary year was 377 or 378 days.
Despite the additional month, though, this system was still not too precise, and the occasional corruption of the pontefices to lengthen or shorten the year (according to the briber's needs) had several legal implications: for example, a contract's expiry date could have been illegally postponed.

Octavian Augustus, wearing
a pontifex maximus robe

Julius Caesar
Important changes were introduced by Julius Caesar in 46 BC (see table below): the length of some months was rearranged, thus avoiding the Intercalans, and obtaining an alternate duration (30/31 days), an easier scheme to follow by farmers; January was chosen again to be the starting month of the year.
Furthermore, having Caesar's astronomers realized that the actual solar year was slightly longer than 365 days, every four years February (already lengthened to 29 standard days) was given one extra day, by repeating the 24th: this was the birth of the leap year.
The reformed system was named Julian calendar, after Julius Caesar; for the same reason, in 44 BC the month of Quintilis was renamed Iulius, in his honour.

the Republican calendar, on the left, compared to the Julian one
(before and after Ottavianus' alteration): all years highlighted are leap years
Towards the end of the 1st century BC, Octavian Augustus, Rome's first emperor, changed the name Sextilis into Augustus, lengthening it to 31 days, probably because 'his' own month had fewer days than Caesar's own. Not to alter the total number of days in the year, the emperor rearranged the length of other months; the result was the same calendar we currently use (see table on the left, right column).

Since February 24th was referred to as ante diem sextum Kalendas Martias ("the sixth day before the Kalendae of March", see DAYS AND WEEKS, further in the page), leap years were named bissexti anni ("years with a double 24th").

The Julian calendar was adopted by the whole western world, and used throughout Rome's imperial age, and the Middle Ages.

Only by the 16th century, astronomers realized that since the solar year did not exactly match the length of 365 + ¼ days, the Julian calendar had been running slightly slow for the past sixteen centuries, losing about ten days.
In 1582 AD, pope Gregory XIII had a correction made, and filled the gap by simply skipping this delay: October 4th (a Thursday) was directly followed by October 15th, counted as a Friday, although according to the sequence of weekdays it should have been a Monday; all days between these two dates did never really exist. Most Catholic countries adopted the change straight away - the new calendar had been, in fact, decreed by a papal bull - while northern European countries did so much later, during the 18th century, and the Soviet Union no sooner than 1918.

Gregory XIII (1572-85)

relief from the tomb of Gregory XIII:
astronomers present their calculations to their pope
To prevent similar situations in the future, a new rule was introduced, by which centennial years would be leap years only when their first two digits are dividable exactly by 4 (this is the case of 1600 and 2000, while 1700, 1800 and 1900 were not leap years).

Counting years, instead, was a later achievement than keeping an account of months. The need of a common parameter for locating major events in time, such as the building of a temple, a battle, a birth date or a death date, etc., came probably during the Republican Age: scholars realized that there were better ways for referring to past events than simply as 'it happened a long time ago'.

Two parallel systems were then introduced.
  • The second criterion was to link the year to the name of an outstanding personage.
    Consuls were appointed in March, the first month of the Republican calendar, so the year itself was usually named either after a consul, or after a general whose victorious campaign was publicly celebrated during that year by holding a ceremony known as a 'triumph'. Stone plates called fasti consulares or fasti triumphales, according to such references, officially marked each day of the year, also mentioning the main public events (religious celebrations, sport competitions, etc.) that had been over the relevant period.
the Fasti hung in the Roman Forum, by the no longer standing Arch of Augustus
(right, a replica in the Museum of Roman Civilization); they listed the names
of all consuls and victorious generals, up to the early Imperial Age

Besides the fasti, other types of calendars were also used:

two sides of the menologium rusticum, from
July to September (left) and from October to December
(replica in the Museum of Roman Civilization)
  • The feriali were similar to the fasti, but included only the festivities of the given period;

  • The menologia rustica were almanacs used for agricultural purposes, in the shape of a cube or a prysm. Their four side faces featured tables that covered three months at a time (i.e. a season), featuring information such as the number of days of each month, the number of daylight hours and nighttime hours, the day on which the Nonae fell (see below), the zodiac sign, the god or goddess whom the month was dedicated to, the main farming duties to be carried out, the main religious festivities; the only specimen of menologium still extant (menologium colotianum, after the name of his discoverer, Angelo Colocci, a secretary of pope Leo X) dates back to the 1st century AD, and is now held by the National Archaeologic Museum in Naples; a second specimen known went lost.

  • Nundinal or market calendars (see below, DAYS AND WEEKS).

  • Astrological calendars, a sample of which is shown in the following paragraph, featured the zodiac signs and were used for knowing the position of the planets.
As previously mentioned, since the introduction of Julius Caesar's calendar, January had been declared the opening month of the year. Over time, this tradition was not preserved everywhere, as during the Middle Ages, although the Julian calendar was still in use, each state followed its own custom. So, for instance, in Spain the year started on the 25th of December, i.e. on Christmas Day, while in England is started on the 25th of March, as well as in Florence, in France on Easter Sunday, in Venice on the first day of March, in Sardinia on the first day of September, and so on. The calendar reformation by Gregory XIII did not affect these local customs, which in some cases persisted even after 1691, the year in which pope Innocent XII officialy declared the 1st of January as the opening day of the Gregorian year.

a nundinal calendar, showing market days
for several cities (list on the right), including Rome
In Rome's early centuries, a common way for measuring short lengths of time was the nundinum: this word literally meant "9 day period", since a market was held in the main cities and towns every ninth day. The actual length of this period, though, was eight days, because Romans counted these days using the so-called 'inclusive numbering', by which the last unit of a series was also considered the first unit of the following series: therefore, the nundinae (i.e. the actual market days) were both the last day of one nundinum and the first day of the following one, as shown below.

In fasti calendars, the days of the nundinum were marked with letters of the Latin alphabet, from A to H.
Furthermore, all days of the year were classified by the pontefices, on the basis of divinations and predictions, as any of the following five types:
  • fasti (favourable), marked "F";
  • nefasti (unfavourable), marked "N";
  • comitiales (assembly days), marked "C";
  • endotercisi (whose central part was favourable, but started and finished as unfavourable days), marked "EN";
  • days marked "NP", whose meaning is still obscure; all the Idus days (see below) and the festivities belonged to this type.
Any public activity could be carried out only on the fasti and comitiales days, and during the central part of the endotercisi days, but not at the beginning nor at the end of the latter, nor at any time during the nefasti days.

detail (July to December) of a fasti calendar of years 20-23 AD:
the first letter tells the nundinal day, followed by the type of day,
and the third column reports celebrations, games, or other activities

detail from the Fasti Praenestini
(early 1st century BC), covering
one full nundinum, from A to H
Besides the nundinal day and the type of day, an average fasti calendar also featured the three main reference days of the month, K (Kalendae), NON (Nonae), EID (Eidus) , spelt in large letters, or small Roman numerals which referred to the count of the days ahead of the next reference to come (see below for a full description), and also the names of major festivities and celebrations, as shown in the following detail.

When additional days were introduced by Julius Caesar's reformation, they were all added at the end of the respective months, and designated as fasti, not to cause further complications.

Meanwhile, the Jewish community, formed by the people taken to Rome as slaves from Palestine, followed a lunar calendar by which cycles of 28 days were divided into four 7 day periods. Gradually, Romans too started indicating days by giving them seven different names. This became a well established custom by the 1st century BC, although the concept of 'week' as a given entity of time was only achieved during the first half of the 4th century AD, under emperor Constantine I.
The Latin names of weekdays were chosen after the planets known by those times; according to the Ptolemaic system, all of them, including the Sun, were believed to revolve around Earth.

dies Saturni day of Saturn Saturday
dies Solis day of the Sun Sunday
dies Lunae day of the Moon Monday
dies Martis day of Mars Tuesday
dies Mercurii day of Mercury Wednesday
dies Iovis day od Jupiter Thursday
dies Veneris day of Venus Friday

Note that the Roman week started in coincidence with the Jewish holiday of Shabbath, the biblical '7th day' on which God had rested, thus believers were not allowed to perform any kind of activity on this day. Romans did not rest on Saturdays, although their dies Saturnii matched the Jewish festivity, what can be told by the alternative Latin name of this day, dies Sabbati ("day of the Shabbath", that is "the day of ceasing from work").

astrological calendar with the days of the week (top row) in the
shape of gods, each of which is identifiable by a symbolic attribute:
 - Saturn (beard and sickle)
 - Sun (rays spreading from his head)
 - Luna (female, moon crescent over her head)
 - Mars (spear and helmet)
 - Mercury (winged helmet and medical staff)
 - Jupiter (beard and thunderbolts)
 - Venus (female, fancy hair-style)
the disk below features zodiac symbols, with the initial letter of their Latin name

The sequence of days, instead, according to historian Cassius Dio, who lived between the 2nd and 3rd century, has an Egyptian origin, and later in time spread to the Roman culture. In order to understand its criterion we should bear in mind the series of planets known by those days (i.e. the Ptolemaic system). Starting with the farthest planet from Earth up to the closest one, the sequence was:

Saturn - Jupiter - Mars - Sun - Venus - Mercury - Moon

The same historian explained the sequence of days with two different schemes:
Only several centuries later, when Rome's leading religion was already Christian, Sunday became the official holiday of the week; after being renamed dies Dominicus ("day of the Lord"), it replaced the Jewish Saturday, which was shifted at the bottom of the series.

The most amazing feature of Rome's calendar system, though, was how days were calculated within each month. In fact, they were not simply numbered, as we do nowadays, requiring rather unpractical calculations.
Each month had three 'fixed days', which bore a specific name: The meaning of Nonae is actually "the ninth day before the Idus", which should be counted keeping in mind the Roman inclusive numbering criterion (i.e. we would now consider this as the eighth day before the Idus).
The name Idus (Eidus), instead, comes from the verb iduare, "to divide", thus meaning "the central day of each month".

The days immediately before and after the aforesaid dates were called their pridie and postridie, respectively. All others were referred to as the number of days before the following fixed ones, by using the complicated expression:

ante diem + the number of missing days + the next fixed date.

For instance, September 9th was called ante diem quintum Idus Septembres ("the fifth day before September's Idus"), in short form: A.D. V ID. SEP.
Another example is May 25th, which was ante diem octavum Kalendas Iunias ("the eighth day before June's Kalendae"), in short: A.D. VIII KAL. IUN., and so on.
The postridie form was not used as much as the pridie, so the general ante diem... form would often take its place: December 6th could have been indicated as postridie Nonas Decembres ("the day following December's Nonae"), but it was more often referred to as ante diem octavum Idus Decembres ("the eighth day before December's Idus"), in short: A.D. VIII ID. DEC.

The following table shows the calendar of a full Roman imperial year (Julian calendar, after the changes by Octavian Augustus).

In naming days, after the expressions pridie, postridie and ante diem, the feminine plural nouns Kalendae, Nonae and Idus required the accusative case (Kalendas, Nonas, Idus), while they were used in ablative case (Kalendis, Nonis, Idibus) on their own 'fixed day'.
These nouns were then followed by the name of the month, used as an actual adjective, matching gender, number and case of the "fixed day" (i.e. feminine plural, in accusative or ablative case).

The old calendar system died out through the Middle Ages. One of its latest specimens can be seen by an ancient church in Rome, called Santi Quattro Coronati, where a hall leading to the church's nunnery has walls covered with 13th century paintings (right), still partly visible, featuring a religious calendar in which the days are marked with letters from A to H, calculated in terms of Kalendae, Nonae and Idus, but in place of the pagan festivities listed in the old fasti, the names of the saint to whom each day was dedicated can be read on the relevant line.
painted religious calendar (13th c.), by the church of Santi Quattro Coronati

this page was created on

(February 29th 2000)